1. Field of the Invention
This invention relates broadly to thrust nozzles for rockets. More specifically, it relates to nozzles capable of varying the cross sectional area of the nozzle throat to help control the magnitude of thrust.
2. Description of the Prior Art
For several reasons, it is desirable to have a dual-thrust capability in certain missiles: (1) On ignition, the missile weighs more than it does after some of the propellant has been burned; and, hence, more thrust is required for lift-off; (2) It requires more energy to overcome the inertia of a rocket in repose and to accelerate it to cruising velocity than it does to maintain cruising velocity; and (3) A missile being launched from an aircraft must clear the launching device as quickly as possible to avoid damage to the aircraft. Hence, the initial thrust of such a rocket must be considerably greater than the thrust required for sustained flight to a distant target.
One means of accomplishing this dual-thrust objective is to design the nozzle throat so that it becomes smaller after its boost phase has been completed. This has the advantages of causing the remaining propellant to burn under a higher pressure, and thus more efficiently; and of providing a much larger nozzle expansion ratio, which uses the thrust of the exhaust plume more effectively in the high-altitude portion of the missile's flight.
Rocket and jet propulsion nozzles that are designed for this purpose are known in the art. For example, U.S. Pat. No. 2,880,576 to Kappus shows a jet propulsion nozzle having radially movable throat elements for varying the diameter of the nozzle throat. U.S. Pat. No. 3,230,704 to Lovingham shows a valve member that moves relative to a thrust nozzle throat; and U.S. Pat. No. 3,182,447 to Bell shows a secondary nozzle member that slides relative to a primary thrust nozzle in response to gases from external sources.
These prior art devices are characterized by being heavy and complex. They depend on precision, moving parts, and require that the entire reaction motor or major parts thereof be specially designed to incorporate their respective inventions. Also, except for the patent to Bell, they are intended primarily for use with liquid propellant systems and are not suitable for use in the more severe environments of high temperature and erosiveness inside solid propellant rockets. U.S. Pat. No. 3,888,419 shows a spike nozzle having a variable throat.